Urban Karst

Definition: The three dimensional, largely hidden, dense systems of urban water networks that include a potentially important network of buried headwater streams give rise to a highly connected network in which groundwater (slow) flows are interspersed with faster groundwater and surface water flows which resemble a karst hydrologic system.

Explanation: Urban landscapes offer streams (often buried in densely developed areas) and a dense surface drainage system for stormwater (e.g., gutters, swales, etc.), but these are also part of a buried, hidden water network which includes sanitary sewers, storm drains, and potable water.  These interact with streams in very complex ways that result in both positive and negative water flows between the engineered, groundwater and natural stream and groundwater systems.

         

Example: The Baltimore stormwater drainage pipe in the headwater portion of the Watershed 263 catchment exhibits constant baseflow, even in dry weather, which is similar in magnitude to a natural forested catchment of similar drainage area.

Figure 1: Underground engineered network pipe types. Left: Storm drains also carry baseflows from groundwater seeping into pipe joints.  Right: Water supply pipes are under pressure and lose large quantities of their flow (up to 30%), increasing groundwater flows. Bottom: Sanitary sewers can both contribute to and take water from groundwater and streams depending on their condition and hydraulic position in the landscape. They can also contribute sewage directly through engineered CSOs, and overflows and shunts to storm drains.

Figure 2: Baseflow in the ultra urban residential Baltimore Street headwater storm drain in Watershed 263.  Left and Lower Right: Baseflow in dry weather in the small headwater storm drain.  Upper right: Manhole access point on Baltimore street; with downtown Baltimore in background.

 

Why Important: These complex interactions between natural and human water systems greatly increase connectivity and impact stream ecosystem structure and function in profound ways.  Examples include the contributions of deicing salt to long term increasing stream chloride trends (over many decades), flows of water between streams and buried sanitary sewer interceptors (and subsequent bacterial pollution), and rising water tables because of leaking potable and sanitary sewage waters. 

Contributed by BES Co-PI Kenneth T. Belt